Acute lymphoblastic leukemia (ALL) is the most common childhood malignancy. It accounts for approximately 25% of all children diagnosed with cancer. During the past 4 decades great strides have been made in the therapy for these children. Induction of a remission occurs more than 95% of the time and intensive, multi-agent therapy has resulted in a 65-70% disease free survival of 5 years. In the past decade the POG began investigating the use of intensive, parenteral methotrexate (MTX) and 6- mercaptopurine (6-MP) as the sole agents in the continuation phase of therapy. The design was based upon the known synergy of the two agents, the potential benefit of sanctuary site penetration by parenteral rather than oral drug and the empiric knowledge that MTX and 6-MP are the two most effective agents for continuation therapy. The preliminary results of ALINC 15 (Phase III) are exciting. For standard risk children the disease free survival (DFS) is 85% and for high risk 70%. The next POG trial (ALINC 16) includes further intensification of both the parenteral and oral components of 6-MP therapy. An understanding of the mechanism of action of the two drugs coupled with the biochemical techniques (e.g. HPLC and radio-ligand binding assays) for studying their pharmacodynamics may allow us to understand why therapy fails some children. The cloning of some of the important enzymes (e.g. dihydrofolate reductase and very recently folylpolyglutamate synthetase) also allows us to possibly find reasons for treatment failure at a molecular level. The goal of our studies is to define a "pharmacologic phenotppe" of either the child or of the leukemic blasts that will predict success, failure or toxicity of treatment with MTX and 6-MP. Blast cell metabolism of MTX will be analyzed at the time of diagnosis and rbc metabolites of both MTX and 6-MP will be analyzed during therapy. At the time of relapse the patient and the leukemic blasts will be studied with reference to MTX and 6-MP metabolism.